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Littoral Hydrovolcanic Explosions: a Case Study of Lava-Seawater Interaction at Kilauea Volcano

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Littoral Hydrovolcanic Explosions: a Case Study of Lava-Seawater Interaction at Kilauea Volcano Joumalof volcanology and geothennal research ELSEVIER Journal of Volcanology and Geothermal Research 75 (1997) 1-17 Littoral hydrovolcanic explosions: a case study of lava-seawater interaction at Kilauea Volcano Tari N. Mattox *, Margaret T. Mangan I Hawaiian Volcano Observatory, U.S. Geological Survey, P.O. Box 51, Hawai'i National Park, H196718, USA ~( Received I October 1995; accepted 6 June 1996 ic 11­ .i­ iI- Abstract c- A variety of hydrovolcanic explosions may occur as basaltic lava flows into the ocean. Observations and measurements lk were made during a two-year span of unusually explosive littoral activity as tube-fed pahoehoe from Kilauea Volcano inundated the southeast coastline of the island of Hawai'i. Our observations suggest that explosive interactions require high 3 entrance fluxes (24m / s) and are most often initiated by collapse of a developing lava delta. Two types of interactions n. were observed. "Open mixing" of lava and seawater occurred when delta collapse exposed the mouth of a severed lava tube re or incandescent fault scarp to wave action. The ensuing explosions produced unconsolidated deposits of glassy lava is 'R fragments or lithic debris. Interactions under "confined mixing" conditions occurred when a lava tube situated at or below In sea level fractured. Explosions ruptured the roof of the tube and produced circular mounds of welded spatter. We estimate a ct water/rock mass ratio of 0.15 for the most common type of littoral explosion and a kinetic energy release of 0.07-1.3 to kJ/kg for the range of events witnessed. or Keywords: littoral cone; explosion phenomenon; lava; pahoehoe; pyrodasts; Kilauea; basalt 0- 3- 1. Introduction advance down the flank of the volcano from active IX: vents on the rift zone and form a system of tubes that Kilauea Volcano has been in near-continuous transport lava to the coastline with minimal cooling. eruption since 1983. For nearly eight years, flows An average volume of 350,000 m3 jday of lava was .00 from this eruption have entered the ocean on the fed through the tube system between 1986 and 1994 accessible south flank of the volcano, providing an 0. Kauahikaua, USGS, unpublished geophysical excellent opportunity to study, at close hand, the data). During this time, 2 km2 of new land was If a liv- interaction between molten lava and water (e.g., added to the island (Fig. 1). JK. Tribble, 1991; Sansone et aI., 1991). Pahoehoe flows In this paper we specifically characterize a period 'lP between 1992 and 1994. During these two years, 101 lava flows entering the ocean in the Kamoamoa area • Corresponding author. Present address: CSIRO, Division of of Hawai'i Volcanoes National Park built a delta 2.9 ael Exploration and Mining, Private Bag, PO Wembley, W.A. 6014, km long and 500 m wide. The evolution of the delta :81. Australia. Tel.: +61-9-387-0786; fax: +61-9-383-7993; e-mail: was distinguished by unprecedented hydrovolcanic om [email protected]. 1 Fax + 808-967-8890; e-mail [email protected]. activity (Table 1). We describe the various types of 0377-0273/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PlI S0377-0273(96)00048-0 2 T.N. Mattox, M.T. Mangan / Journal of Volcanology and Geothermal Research 75 (J997) 1-17 .....'"~ ~ Z N S <r:: 0 ~ [2 U 0 U e; 0 u <r:: t:l.. 0/" ;, -\g( 'N TN. Mal/ox, M. T Mangan / Journal of Volcanology and Geothermal Research 75 (1997) /-/7 3 Fig. 2. (a) Aerial view of a prograding lava delta. ote the diffuse plume of white steam at the leading edge of the delta, indicative of multiple small streams pouring into the ocean. The delta is approximately 500 m wide and extends about 200 m into the ocean in this image. (b) Aerial view of a mature lava delta. The delta has begun to collapse into the ocean, forming a lava bench that is separated from the main delta by an ocean-facing scarp. Note the consolidated steam plume, evidence of focused entry of lava into the ocean. The bench is approximately 350 m wide and extends about 60 m into the ocean in this image. \ Fig. I. Map of lava flows from Kilauea's East Rift Zone eruption. This map shows the dates and distribution of flows from the major vents erupted between 1983 and 1994. Most of the material erupted since 1986 has entered the ocean along a 12-krn-Iong stretch of the southeast coast of Hawai'i. The fonner coastline is shown for reference. The contour interval is 200 feet. 4 T.N. Mattox, M.T. Mangan j Journal of Volcanology and Geothermal Research 75 (1997) 1-17 event observed, and identify some of the physical submarine bathymetry and the volume flux of lava factors constraining the style and intensity of explo­ entering the ocean (Hon et aI., 1993). The two sions. Conditions required to initiate littoral explo­ largest deltas built during this eruption filled small sions on pahoehoe flows are compared in a general bays at Kalapana and Kamoamoa (Fig. 1). During way with hydrovolcanic activity associated with ad­ the first few days of construction, these deltas grew vancing 'a'a flows. at rates of - 38,500 and - 18,500 m2/day, respec­ tively (Fig. 3). Both continued to advance in the following weeks, but at reduced rates as the leading 2. Littoral setting edge of the delta moved beyond the shallow waters of the coastline and encountered steep offshore When pahoehoe flows first reach the ocean, the slopes. The developing tube system "caught up" interaction is relatively quiescent. Flow lobes drip with the active delta front as growth slowed, and the over old sea cliffs or spread out along established lava streams flowing into the ocean consolidated into beaches. The lava is passively quenched as it enters a single, well-defined tube entry. It is at this time the surf zone and shatters to glassy blocks and lapilli. that the leading edge of the delta becomes prone to These fragments build a loose submarine debris slope. catastrophic collapse. Geodetic monitoring of active Later flows build out on this slope to form lava lava deltas has revealed subsidence rates of several deltas (Figs. 1 and 2a,b), which eventually can ex­ centimeters per month (Kauahikaua et aI., 1993). tend hundreds of meters seaward (Moore et aI., Subsidence of the delta is coincident with inflation 1973; Mattox, 1993a,b). As the flow field matures, a of lava tubes (Kauahikaua et al., 1993) and the tube system is established within the delta, and lava development of large cracks parallel to the coast. enters the ocean at a few discrete points. Typical The unstable front of the delta, or lava bench, is volume fluxes of 2-5 m 3Is flow through tubes 1-3 bounded inland by an ocean-facing scarp (Fig. 2b m in diameter Oackson et aI., 1988; Heliker et aI., and Fig. 4A). This scarp can be either a pre-existing 1993; Kauahikaua et aI., 1996; J. Kauahikaua, USGS, sea cliff or a new failure surface. Lava benches on unpublished geophysical data). Tubes at the leading the Kamoamoa delta were separated from the main edge of the delta often reside at, or below, sea level delta by a 1- to lO-m-high scarp. The benches were 0. Kauahikaua, USGS, unpublished geophysical typically elliptical and extended up to - 200 m data). along the coastline and - 40 m seaward. They The rate of delta formation depends largely on collapsed repeatedly during the evolution of the delta. ooסס25 • ooסס20 ...~ ! 150000 ~ • Kealakomo Delta oo oסס10 .s I!. ~ I!. Kalapana Delta •• • Kamoamoa Delta 0~~--'--1,-L--~-"--+~~-+-~~+--"-~~f--"-~-'--+~~'---+-~--'--"--i o 5 10 15 20 25 30 35 40 Days since first entry into ocean Fig. 3. Growth rates for the Kamoamoa and Kalapana deltas, with Kealakomo delta (Mauna Ulu, Moore et aI., 1973) for comparison. Lava flows entering the ocean during the Mauna Diu eruption of Kilauea built a delta at a rate of 6000-9000 m2 jday (Moore et aI., 1973). The lower rate of growth is a reflection of steeper off-shore bathymetry. Both the Kamoamoa and Kalapana lava deltas were formed in shallow bays; their growth rates declined once the flows reached steeper submarine slopes. A. Active lava delta, bench and tube LAVA DELTA New surface flows LAVA BENCH I Sea Level 20m B. Open mixing: complete collapse of C. Confined mixing: partial collapse of lava lava bench severs active lava tube bench submerges and fractures a portion of active lava tube Tephra Jets Littoral Lava Fountains Lithic Blast Waves impact tube mouth and hot scarp a Level Lava + Water ~..... ~ Sea Level ::::!..... .....I 'I Fig. 4. Hypothetical cross section of a lava delta and bench showing the location of littoral hydrovolcanic explosions. (A) Hyaloclastites formed at the ocean entry build a submarine debris slope that is subsequently capped by pahoehoe flows. Lava tubes on the bench can reside below or at sea level, due to continuous subsidence of the delta (Kauahikaua et aI., 1993). (B) Profile of the front of the delta immediately following a complete bench collapse. (C) Profile of the front of the delta immediately following a partial collapse of the bench. 6 T.N. Mattox, M. T. Mangan / Journal of Volcanology and Geothermal Research 75 (1997) 1-17 During most collapses, the bench slumped into the that attains heights ~ 40 m. Jets are highest immedi­ ocean, severing the lava tube. Occasionally, the bench ately following the collapse and diminish with time. subsided abruptly but did not break away.
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